Knowledge of diluent gas components and their concentrations in natural gas is of significant importance to the handling, transmission, and custody transfer of the gas. This knowledge is of primary importance with respect to processed natural gas transmitted and distributed nationwide for sale and consumption. It is also important with respect to natural gas in its raw form recovered at the wellhead (production gas) to identify the gas composition entering the processing plant and to account accurately for custody transfer pricing between gas industry producers and processors. The resulting processed gas, properly adjusted to a normal range of acceptable commercial composition, enters a network of gas transmission trunk pipelines that bring together several independent sources of processed gas from different geographical regions and thereafter that gas is distributed via smaller and more numerous pipelines to municipal, industrial, and residential consumers. At many points in this gas transport and transfer network, custody transfer between owners and customers occurs wherein the thermal energy content and quality of the gas must be determined in order to set the selling price and to establish that the transported gas mixture is in compliance with regulatory standards. These requirements are of major importance to the economics and business enterprise within the gas industry. Therefore, as natural gas industry transport and custody transfer operations expand, more efficient and more widely distributed capabilities are needed for monitoring the gas energy content and diluent gas components in processed natural gas. In particular, these monitoring techniques must be accurate, reliable, safe, and capable of near-real-time operation as well as inexpensive enough for cost-effective widespread installation and use.
One present-day method used for determining the heating value (thermal energy content) of processed natural gas employs a natural gas chromatograph instrument to measure the various hydrocarbon constituents and diluent gas constituents, wherein the latter are primarily carbon dioxide and molecular nitrogen. The diluent gases are typically present in amounts of only a few percent by volume and are required to be less than a total concentration of six percent by federal regulatory limit. Gas analysis data from the gas chromatograph are used to calculate the heating value of the gas using established thermodynamic relationships. However, the cost of gas chromatograph installations and their operation is sufficiently high that their widespread use is prohibitive.
Other methods for assessing natural gas energy content are in various stages of research and development. Some of these methods are “inferential” and are aimed at measuring certain inherent thermodynamic and physical properties of the gas which, by correlative analysis, can be collectively interpreted to yield an accurate and unambiguous value of the gas heating value. One such method has recently been demonstrated to yield gas energy measurements to within the same accuracy generally attributed to gas chromatograph determinations. This inferential method requires input information on the pressure, temperature, speed of sound, carbon dioxide concentration, and molecular nitrogen concentration in the gas in order to determine the thermal heating value. This method is described in U.S. Pat. No. 6,604,051 B1, entitled “System and Method to Determine Thermophysical Properties of a Multi-Component Gas” and U.S. Pat. No. 6,704,660 entitled “A System and Method to Determine Thermophysical Properties of a Multi-Component Gas at Arbitrary Temperature and Pressure”, both to K. Behring and T. Morrow.
Present-day sensor technology is available for directly measuring the pressure, temperature, speed of sound, and carbon dioxide parameters needed for the energy analysis. However, because of its stable chemical nature, there are no existing methods for directly sensing molecular nitrogen. Consequently, to fully implement the inferential method of natural gas energy determination, a means for determining the molecular nitrogen diluent content is required.